JP 2003-232674 discloses one example of a sensor for detecting vibrations produced by a roller bearing. A diagnostic device determines abnormalities caused by bearing damage based on evaluation of the frequencies of the vibrations.
U.S. Pat. No. 6,672,681 discloses a vibration-detecting sensor on a railway wheel unit. Output signals are transmitted to a processing unit which is located on board the railway car or the train to signal an incipient fault and/or damage state of the wheel unit.